Satellite Operation

QuikSCAT operates in a near polar orbit. Its angle of inclination is about 98 degrees, which means that according to a compass it is flying north by north west at a directional angle of 352 degrees when it is ascending (i.e. south to north). It flies in a circular orbit at an altitude of approximately 800 km above sea level. It completes a full orbit in about 101 minutes, which translates to a little more than 14 orbits per day.

Seawinds is the main instrument on the QuikSCAT satellite. It actively transmits 13.4 GHz microwave pulses at a rate of 1 pulse every 5.4 ms. The pulses are alternately polarized, vertical and horizontal. It uses the same parabolic antenna for both pulses, with different feeds. The antenna rotates at a rate of 18 rotations per minute. The feeds on the antenna are set up so that the vertical polarized beam has an elevation angle of 45 degrees and the horizontal polarized beam an elevation of 39 degrees. This creates an outer and an inner beam.

As the antenna rotates, the instrument pulses form a circular footprint on the ground. The outer beam covers a circle on the surface of 1800 km diameter while the inner beam covers about 1400 km. When the circular rotation of the antenna is combined with satellite movement of approximately 25 km per rotation, a helical shape is trace out on the ground. This type of coverage pattern allows the instrument to cover 90 percent of the earth every day.

A fixed point on the ground, such as the CGS, will observe the satellite differently depending upon where the satellite is in the sky. If the ground point is directly in line with the satellite's track, it sees four beam crossings; the outer beam then the inner beam as the satellite approaches, followed by the inner beam then outer beam as it departs. The timing between the four beam crossings depends upon the distance from the ground point to the track of the satellite. IF the ground point is far enough to the side of the satellite's track it will not see the inner beam and only observe the outer beam approach and depart. This is referred to as a 'two beam' crossing, or 'outer beam only' crossing.

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